1. Field of the Invention
This invention relates generally to electronic amplifiers with semiconductor amplifying devices, including a signal feedback path having negative feedback, and more particularly, a signal feedback path for increasing slew rate.
2. Description of the Related Art
An operational amplifier (op-amp) having a class “B” or “AB” buffer or output stage is well known. The equations that govern the operation of an op-amp are also well known. If the op-amp is to be made stable, the unity gain crossing frequency must be controlled. The conventional technique for controlling the unity gain crossing frequency is to couple a compensation capacitor C in a feedback path between an output and an input of the op-amp. The well known Miller capacitance reduces the gain of an amplifier by providing negative feedback between the base and collector of bipolar transistors. A compensation capacitor C is a capacitor, external to the transistor(s) of the op-amp and possibly also external to the integrated circuit of the op-amp, which acts as an additional Miller capacitance.
If more than two poles exist before the unity gain frequency fUNITY is reached, the op-amp is unstable as a result of a 90 degree phase shift that occurs in the feedback path due to inherent characteristics of each filter of the op-amp. More phase shifts are produced at higher frequencies. At frequencies having more than two phase shifts, a desired negative feedback becomes an undesired positive feedback, and the op-amp becomes unstable. The compensation capacitor C operates to push the first dominant pole down to a low enough frequency such that the op-amp gain falls below unity before the second pole is reached. The larger the compensation capacitor, the larger the feedback, thereby lowering the gain at all frequencies. Because the gain decreases as frequency increases, a designer wants the gain to fall to unity prior to reaching a frequency at which the feedback becomes positive. The larger the value of the compensation capacitor, the more stable is the op-amp. However, the value of the compensation capacitor C also affects the maximum slew rate. It is also desirable to have a high slew rate, which requires a small value of the compensation capacitor C. Reducing the value of the compensation capacitor C however, makes the op-amp less stable because the first dominant pole will then be moved to a higher frequency. Therefore, a high value of the compensation capacitor C, for greater stability, and a low value of the compensation capacitor C, for higher slew rate, conflict with each other.
The slew rate is defined as the maximum rate of change in output voltage, ΔVO/Δt, for large input signals. If a step function is applied to the input at time=tO, a small signal analysis of the op-amp circuit predicts a response, VO, that rises exponentially to about VCC at time=t1. However, the actual measured response rises linearly to about VCC at a later time of time=t2. The slower response occurs because the step function is a fast and a large signal change at the input that requires a fast and large charging and/or discharging of circuit capacitance, including the compensation capacitor C, which must be accomplished using the available bias current at a stage driving the class AB output stage. When the available bias current cannot charge/discharge circuit capacitance fast enough to give the predicted small signal analysis response, the op-amp is said to be slew rate limited by the available bias current. When the input to op-amp changes at a rate faster than the maximum slew rate, the op-amp does not respond linearly, rather the op-amp delivers as an output whatever current is available. The slew rate is expressed in volts per microsecond (V/μs).
An op-amp comprises at least a transconductance differential input stage and a voltage amplification output stage. The frequency where the open loop gain falls to unity for op-amp is:fUNITY=gm1/C  Equation (1)where gm1 is the transconductance of the input stage. For a bipolar transistor, and assuming no emitter resistors in the differential input stage, gm1 is related to the tail current ITAIL shared by the input stage transistors, by:gm1=ITAIL/2VT  Equation (2)
A typical prior art two-stage op-amp 100 is shown in FIG. 1. The op-amp 100 comprises an input stage 110 and an output stage 120. The input stage 110 comprises a differential pair of transistors 101 and 102. Transistors 103 and 104 form an active load for transistors 101 and 102. The output stage 120 produces a large voltage gain. A negative feedback path 125 comprising a compensation capacitor C 106 is coupled from an output node 111 of the output stage 120 to an input node 113 of the output stage 120. The value of the compensation capacitor C 106 and amount of tail current ITAIL 108 are determined by the small signal characteristics of the op-amp 100, which, in turn, are chosen based upon how the op-amp is to be used. In particular, the value of the compensation capacitor C 106 is set to a large enough capacitance to make the op-amp stable at the intended operating frequency.
The slew rate is determined by the value of the compensation capacitor 106 and the amount of tail current ITAIL 108. The slew rate measures how fast the op-amp can charge a load capacitance CLOAD 115, such as a liquid crystal display (LCD) panel. Although, the slew rate may be affected by the amount of the capacitance of the load CLOAD 115, the load capacitance is usually fixed. For a given load, if the output stage's maximum output current IO 112 is larger than the current necessary to charge the load capacitance (which is usually the case), then the slew rate is:Slew Rate=ΔVO/Δt=ITAIL/C  Equation (3)That is, the slew rate is determined by the maximum current available in the stage driving the output stage.
It can be shown that for a MOSFET,
      gm    1    =                              I          TAIL                2            ⁢      μ      ⁢                          ⁢              C        ox            ⁢              W        L            For an op-amp comprising MOSFETs, it can also be shown that
                              Slew          ⁢                                          ⁢          Rate                =                                            gm              2                                      C              LOAD                                ⁢                                                    2                ⁢                                  I                  TAIL                                                            μ                ⁢                                                                  ⁢                                  C                  ox                                ⁢                                  W                  L                                                                                        Equation        ⁢                                  ⁢                  (          4          )                    where gm1 is the transconductance of the input stage 110 and gm2 is the transconductance of the output stage 120 of the op-amp 100 of FIG. 1. Therefore, the slew rate of the prior art MOSFET op-amp 100 could be improved by increasing the differential pair's bias current, ITAIL. However, because ITAIL is under the radical in Equation (4), in order to double the slew rate, the differential pair's bias would have to be disadvantageously increased by a factor of four. Increasing the bias current ITAIL of the differential stage would disadvantageously increase the quiescent current of the op-amp. Moreover, increasing the bias current of the input stage of an op-amp disadvantageously increases the bandwidth of the op-amp. Furthermore, the small signal performance would become degraded because the small-signal voltage gain increases in proportion to the current ITAIL, but gm1 increases in proportional to the square root of the current ITAIL, i.e., more slowly.
For an op-amp comprising bipolar transistors (not shown), the slew rate is completely determined by the small signal design and the op-amp's required quiescent current. In particular, ITAIL is also determined by the small signal design of the op-amp. For an op-amp comprising bipolar transistors, it can be shown that the slew rate is independent of gm1, and that the slew rate cannot be increased by increasing ITAIL because increasing ITAIL requires a corresponding increase in C to maintain stability.
To obtain higher slew rates, designers of prior art circuits have: a) reduced the input stage transconductance gm1 associated with the input stage by adding resistors in the emitter paths of the input stage transistors, and then increased ITAIL or decreased the value of the compensation capacitor C to achieve stability, but adding resistors to the emitter paths of the input stage transistors produces additional noise; b) decreased the value of the compensation capacitor C, but this is undesirable because this adversely affects the small signal parameters; c) increased the current available to charge the compensation capacitor C, but this is disadvantageous in battery-powered devices; or d) used cross-coupled transconductance reduction, but this increases circuit complexity. A more desirable alternative is to design the circuit for desired small signal parameters and then modify the circuit to change the slew rate under large signal conditions in a way that overcomes the disadvantages of the prior art.
Examples of known circuits related to improving slew rate, or related to driving LCD panels are: U.S. Pat. No. 4,320,347, issued Mar. 16, 1982, to Haque, entitled Switched Capacitor Comparator discloses voltage comparator that has two periods of operation, and in which a feedback capacitor is switched into the circuit during an initialization period to ensure that the op-amp is stable while in a unity gain mode, and then switched out of the circuit during a period in which the op-amp is operating as a voltage comparator. However, during the period that the voltage comparator is operating as an amplifier, the feedback capacitor is not switched in and out of the circuit; furthermore, Haque lacks a clamp to shunt the feedback capacitor to ground.
U.S. Pat. No. 4,500,846 issued, Feb. 19, 1985, to Lewyn, et al., entitled Circuit for Effecting Improved Slew Rate of Operational Amplifiers discloses an op-amp with an input stage and an output stage and two compensating capacitors connected in parallel that are switchable into and out of a feedback path, controlled by an external clock pulse train, to vary the amount of frequency compensation. However, Lewyn et al., lacks a clamp to shunt either compensation capacitor to ground. As a result, Lewyn et al., disadvantageously requires the input stage to fully charge/discharge the compensation capacitor.
U.S. Pat. No. 5,416,442 issued, May 16, 1995, to Hobrecht entitled Slew Rate Enhancement Circuit for Class A Amplifier discloses an amplifier that has one capacitor in each of two separate feedback paths and means to switch the capacitor of one of the feedback paths in and out of its feedback path. However, Hobrecht lacks a clamp to shunt any compensation capacitor to ground.
U.S. Pat. No. 5,471,171 issued, Nov. 28, 1995, to Itakura et al., entitled Amplifier Device Capable of Realizing High Slew Rate with Low Power Consumption discloses an amplifier for use in driving a LCD display. Itakura et al., uses complex circuitry to realize a high slew rate by increasing a total bias current when needed. However, Itakura et al., lacks any means of switching a compensation capacitor in or out of the circuitry.
U.S. Pat. No. 5,825,250 issued, Oct. 20, 1998, to Tomasini et al., entitled Operational Amplifier having an Adjustable Frequency Compensation discloses an op-amp have two compensation capacitors in parallel that are switched in or out of a feedback path by an external signal generated by a logic circuit, to vary the amount of frequency compensation. However, Tomasini et al., lacks a clamp to shunt any compensation capacitor to ground. As a result, Tomasini et al., disadvantageously requires the input, or transconductance, stage to fully charge/discharge the compensation capacitor(s).
U.S. Pat. No. 6,333,674, issued Dec. 25, 2001, to Dao, entitled Feedback Stabilization Apparatus and Methods discloses an audio amplifier having a parallel LC tuned circuit in which additional capacitance is switched into the LC tuned circuit, by diodes, as the voltage applied to the LC tuned circuit increases. However, the capacitance that is switched is not compensation capacitance.
U.S. Pat. No. 6,392,485 issued, May 21, 2002, to Doi, et al., entitled High Slew Rate Differential Amplifier Circuit discloses a differential amplifier with two compensating capacitors in parallel, used for driving a LCD panel. However, Doi, et al., lacks any means for switching either capacitor in or out of the circuit.
U.S. Pat. No. 6,456,161 issued, Sep. 24, 2002, to Smith, entitled Enhanced Slew Rate in Amplifier Circuits discloses an amplifier having a pair of compensation capacitors, but neither of them are switched in or out of the circuit.
Thus, what is needed is a circuit that overcomes the disadvantages of the prior art, and that increases slew rate capability without degrading other performance parameters. In particular, what is needed is an uncomplicated circuit that maintains stability at the desired frequency under small signal conditions, and increases its slew rate under large signal conditions without also increasing tail current under small signal conditions, and having a sufficiently high slew rate useful for driving a LCD display panel. These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds.